1. Field of Invention
The invention relates to a process for the preparation of aromatic hydroxy carboxylic acids or salts thereof by reacting a corresponding alkali metal phenolate or naphtholate with carbon monoxide and an alkali metal carbonate.
2. Description of the Related Art
Aromatic hydroxy carboxylic acids such as 2-hydroxy-naphthalene-6-carboxylic acid are important intermediates, for example, in the production of dyes, polyesters, textile assistants and pharmaceuticals (see, for example, EP-A 0 292 955 and U.S. Pat. No. 4,393,191).
In industry, compounds of this kind are often prepared by the Kolbe-Schmitt reaction, i.e. by reacting a corresponding alkali metal phenolate or naphtholate with carbon dioxide (see e.g. EP-A 0 327 221 or U.S. Pat. No. 4,966,992). However, there is still room for improvement in the yields of these processes.
An alternative process for the preparation of aromatic hydroxy carboxylic acids is described in GB 1 155 776 (.sub.= U.S. Pat. No. 3,655,744). In this process specific aromatic alkali metal phenolates or alkaline earth metal phenolates or naphtholates are reacted with alkali metal or alkaline earth metal carbonates, carboxylates or dicarboxylates in the presence of carbon monoxide. The yields and, in particular, the selectivities of the process are likewise not adequate for all areas.
It was therefore desirable to improve the process described in the British patent application in respect of the yields and the selectivity of the reaction. It was also desirable to design the process in such a way that it can be carried out continuously in a simple manner.
Although GB 1 155 776 mentions the possibilities of a continuous reaction regime, the addition of solvents and the infeed and takeoff of solids as a suspension, it does not give concrete examples.